Sanyo Semi Hermetic Compressor Problems

[dallas]

Help, Any assistance would be most appreciated.

I have a 6 cylinder Sanyo semi hermetic compressor serving a relatively large DX air conditioning plant on R22. The plant has 3 coils (2 of 50kW and 1 of 100kW).

We have a new / reconditioned compressor which has been recently installed as a direct replacement to the existing compressor. The compressor has 4 stages of unloading.

We start up the compressor and it operates fine with the unloaders operating as required by suction pressure. After the compressor has been operating for approximately 1 hour, the suction pressure rises and the head pressure falls as if the compressor is losing compression. The unloaders are still energised and the compressor maintains 200kPa oil pressure.

Our compressor rebuilder first suggested that there was an issue with the unloading mechanisms. They replaced the base plate and made the first stage of unloading permantently on. When we restarted the compressor, after approximately 1 hour of operation, the compressor did exactly the same thing.

The compressor rebuilder then pulled off the heads and inspected the heads and the rings, however nothing obvious was amiss.

We have now put the compressor back together and will restart in the morning, however as nothing has been identified, I think the same thing will happen again.

Any suggestions as to what is going on?

We are keeping all our coils open at all times, and have confirmed our superheat at each individual TX Valve. Nothing is amiss in the refrigeration system as far as I have tested.

It is as if there is a pressure relief valve in the compressor which is equalising the compressor. The compressor rebuilder is saying that there is no such valve present in this compressor. The head pressure whilst the compressor is operating correctly is as expected around 1,500kPa and does not go up.

There is another compressor which basically runs in parallel with this system (totally seperate refrigation system, identical air systems and condenser water). This system is running fine and not missing a beat.

Any help would be most appreciated as I don't know what to do from here. I am sure the problem is compressor related, however I don't have enough knowledge of this compressor to resolve.

[Brian_UK]

No gas bypass valves in the system?

[lowcool]

it may well be possible that comp is slowing down,have you taken any current draws it may be nipping up

[Gary]

Sounds like an unloader/control problem to me.

[Magoo]

A six cylinder compressor with four stages of unloading, is it unloading individual cylinders or banks of cylinders and the last stage stopping.
magoo

[dallas]

the unloading is set up as 2 cylinders, 1 cylinder, 1 cylinder, 2 cylinders.

We have it running just on the first stage which has been bypassed on the unloader plate. i.e. 2 cylinders are permantely loaded.

The compressor operates for up to one hour, 6k superheat at the compressor, 6k superheat at the three TX valves.

With no change in any of the refrigeration parameters, the compressor unloads and the suction pressure rises and the discharge pressure falls. The oil pressure remains the same at 300kPa throughout everything.

There is something in the compressor which is causing the compressor to unload. Does anyone know what this could be?